1. Field of the Invention
This invention relates to a multi-beam klystron apparatus for amplifying radio-frequency power.
2. Description of the Related Art
The conventional klystron apparatus includes an electron gun unit for generating an electron beam, an input unit for inputting radio-frequency power, a radio-frequency interaction unit for amplifying the radio-frequency power by the interaction between the electron beam and the radio-frequency electric field, an output unit for outputting the radio-frequency power from the radio-frequency interaction unit, a klystron body having a collector unit for capturing the used electron beam having passed through the radio-frequency interaction unit, and a focusing magnetic field unit mounted on the klystron body for focusing the electron beams. The radio-frequency interaction unit includes drift tubes through which the electron beams pass, an input cavity connected to the drift tubes along the direction in which the electron beams proceed and a plurality of intermediate and output cavities, wherein the input cavity is connected with the input unit and the output cavity with the output unit.
FIG. 10 shows the result of analyzing the lines of magnetic force of a focusing magnetic field unit of the single-beam klystron apparatus. In many actual cases, a focusing magnetic field unit 1 includes several to ten and several electromagnets 2a, 2b arranged along the collector unit from the electron gun unit side of the klystron body and a magnetic pole 3 having an electron gun-side pole piece 3a, a collector-side pole piece 3b and a return frame 3c. In this focusing magnetic unit 1, the electron beam is focused by the magnetic field generated by the current supplied to the electromagnets 2a, 2b. In FIG. 10, the electron gun unit is arranged on the lower side, and the collector unit on the upper side. Reference numeral 4 designates lines of magnetic force, which are too thin and not shown in the magnetic pole 3.
Generally, the electron beam of the klystron apparatus, in the absence of radio frequency, has a substantially constant thickness. In the radio-frequency operation, however, the electron beams are bunched progressively downstream in the direction of radiation and, in the neighborhood of the output cavity, the degree of density thereof comes to be clearly defined. At points where electron density is high, the electron beam tends to spread diametrically due to the reaction of the electrons due to the space charge thereof. For this reason, a method is employed in which the radius of the drift tube surrounding the electron beam is increased to prevent collision or the axial magnetic flux density of the focusing magnetic field in the neighborhood of the output cavity is increased to suppress the spread of the electron beam. The method of simply increasing the radius of the drift tube, however, encounters the problem of a reduced output conversion efficiency, and therefore a method is generally employed in which the axial magnetic flux density of the focusing magnetic field is increased in the neighborhood of the output cavity.
FIG. 11 is a graph showing the relation between the axial position from the cathode (position 0 of the distance Z) of the electron gun unit of the single-beam klystron apparatus and the axial magnetic flux density. The magnetic field is formed in the same direction from the cathode of the electron gun unit to the collector unit, and the axial magnetic flux density is 680 Gauss in the neighborhood of the input cavity while it is 820 Gauss, or 20% higher, in the neighborhood of the output cavity. The electron beam is focused in such a manner as to be wound on the lines of magnetic force and therefore an effective means for preventing the dispersion of the electron beam is provided by increasing the axial magnetic flux density in the neighborhood of the output cavity with the electron beam more bunched.
FIG. 12 is a graph showing the relation between the axial position (distance Z) from the cathode of the electron gun unit of the single-beam klystron apparatus and the lines of magnetic force at the radius R in the neighborhood of the center axis. It is understood that the electron beam having the radius indicated by the second lowest line, for example, proceeds along the lines of magnetic force and therefore the radius thereof is reduced from 7 mm in the neighborhood of the input cavity to 6.3 mm in the neighborhood of the output cavity.
Also, it is generally known in this particular field of technique that the lower the ratio of the beam current to the beam voltage called the perveance, the higher the output conversion efficiency of the klystron apparatus. Also, one of the means for improving the efficiency is known to be provided by a multi-beam klystron apparatus in which the number of electron beams is increased from one to several or several tens and the perveance of each electron beam is set low to suppress the beam voltage applied to the electron gun unit while at the same time improving the overall output conversion efficiency (Jpn. PCT National Publication No. 2002-520772).
In the multi-beam klystron apparatus, several to several tens of electron beams are arranged at a distance from the center axis of the klystron apparatus. For example, electron beams are arranged at intervals of 60 degrees at the distance of 60 mm from the center axis of the body of the klystron apparatus.
In this multi-beam klystron apparatus, an increase in the axial magnetic flux density in the neighborhood of the output cavity to suppress the spread of the electron beam, like in the single-beam klystron apparatus, would pose the problem that the lines of magnetic force are curved and so are the electron beams. This is specifically explained with reference to the graph of FIG. 13 showing the relation between the axial position (distance Z) from the cathode of the electron gun unit of the multi-beam klystron apparatus and the lines of magnetic force at the position (radius R) from the center axis of the klystron body in the neighborhood of each electron beam. In the case of the electron beam having the center axis indicated by the second lowest line, for example, the center axis of the electron beam is located at the distance of 60 mm from the center axis of the klystron body in the neighborhood of the input cavity, while the center axis of the electron beam is moved to the point at the distance of 54 mm from the center axis of the klystron body in the neighborhood of the output cavity, thereby curving the electron beam. Under this condition, the electron beam would impinge on the drift tube and therefore it is impossible to assure stable operation of the multi-beam klystron apparatus by increasing the axial magnetic flux density in the neighborhood of the output cavity.
In the case where the output unit such as the waveguide or the coaxial tube output unit connected to the output cavity is led out substantially at right angles to the center axis of the klystron body, on the other hand, a focusing magnet may not be arranged at the particular location. In such a case, the axial magnetic flux density is reduced in the neighborhood of the output cavity. This curves the lines of magnetic force at other than the center axis of the klystron body, with the result that the electron beam is curved in the multi-beam klystron apparatus in which the electron beam passes a point distant from the center axis of the klystron body.